The present invention relates to the storage of fluids and in particular to the storage of fluids in a system of modular tanks.
The need for storage of water is well known, especially in the west where many regions are arid or semi-arid. Water may be stored for a variety of reasons: potable water for household use; water for livestock; and fire cisterns are some of the more common applications. While open, above ground storage is common for livestock, enclosed storage is preferred for potable water in order to avoid contamination. Where enclosed tanks are used, underground tanks are often preferred. By burying the tank, the tank itself is supported, the tank is protected, and the contents of the tank are insulated from temperature changes. Underground tanks are also commonly used as septic tanks where municipal sewer service is not available.
Various materials have been used to construct underground storage tanks. Steel and concrete have been in use for decades. Fiberglass is a newer material rapidly gaining popularity especially in petroleum storage. Steel tanks are prone to rusting, especially where they are exposed to ground water. Concrete tanks do not rust, but are semi-porous and will deteriorate with time. Fiberglass has good resistance to corrosion, but is relatively brittle, requiring careful handling, especially during installation. A sharp blow or inadvertent contact with the installation equipment can easily damage a fiberglass tank.
Both steel and concrete tanks are relatively heavy. This typically results in the tanks being constructed relatively near the point of installation to reduce transportation difficulties and expense. This weight also effectively limits the maximum size of tank which can be constructed of these materials. Fiberglass is a much lighter material and can be used to fabricate a tank which is relatively rigid for its size. This enables the construction of relatively large, light weight tanks which are efficient to build and transport. However, there must be sufficient room for the necessary equipment, such as a crane, at the job site to off load and install the tank. Additionally, construction of fiberglass tanks is a labor intensive process which makes them relatively expensive.
Polyethylene has several characteristics which make it particularly suitable for potable water storage. It is typically formed into tanks using a rotary molding process which results in a one piece, seamless tank. This limits opportunities for bacterial growth and ground water infiltration. The material is impact resistant, flexing rather than cracking like fiberglass, and is highly corrosion resistant. However, this flexibility results in a vessel which is structurally weaker, limiting the size of the tank which can be cost effectively constructed. Large radiuses and flat surfaces are prone to buckling and collapse when the surrounding ground shifts or freezes, or from external pressure due to ground water.
Where access to the site is restricted, none of the above approaches is ideal. This is a common situation in mountain communities where the terrain limits access. Roads are often narrow and sharply curved. Surfaces may be rough and unimproved, with dirt or gravel being common. However, houses and ranches in these regions are often the ones most in need of water storage because municipal water is unavailable, natural water supplies may not exist, and wells are difficult and expensive to drill and may provide only a minimal flow. In such a situation, a fire cistern may be needed for safety and is often a prerequisite to obtaining fire insurance.
Compounding the problem is the fact that the required storage capacity often exceeds the capacity of any single tank which can be efficiently moved to the site and installed. It thus becomes necessary to interconnect several tanks to form a storage system of sufficient capacity. Several designs for such systems are known. Most common are tanks which couple together with mating pipe flanges. Where the terrain is uneven, achieving the required alignment to allow insertion of all of the bolts which join the flange can be very difficult. If it is achieved, shifting of the tanks due to settling can result in significant stresses in the tanks. The strict alignment requirements can increase the cost of the excavation as it must be matched to the tank, there being little adaptability in the tank. The level of skill required to properly install and join these types of tanks is often not available in remote areas, requiring bringing in skilled labor for the installation. Further, supplies or materials may be required which are not commonly available. If additional items are needed, the project may be delayed while they are brought in.
Where the tank material is polyethylene, manufacturing characteristics of the material introduce additional problems. Uneven cooling of the mold and relative time of release of various portions of the tank from the mold cavity cause warping and distortion in the tank which can vary from one unit to another. While not significant to the performance of the tank, these variations can easily be large enough to make it difficult to align all of the bolts for an interconnecting flange, and nearly impossible to align multiple connections on the face of a tank.
One solution to the alignment problem is to interconnect the tanks with flexible hose or tubing This allows for misalignment between the tanks and eliminates the flange mating problem. However, the joints used for this type of connection are prone to leaking and require periodic inspection and repair, which typically require at least partial excavation. Additionally, the hardware components of the joint are external to the tank and are exposed to the environment and backfill material both during and after installation.
The above discussion is also applicable where above ground storage of water or other fluids is needed. Transport of full size tanks may not be practical and on-site construction difficult.
Vehicle mounted tanks pose a related problem, especially where the tank is not permanently mounted. A tank may be installed and removed with relatively high frequency as needs dictate. Further, where water transport is relatively uncommon it may not be cost effective to dedicate a vehicle to water transport. However, typical tanks are constructed of steel and are difficult to install and remove without specialized equipment such as a crane or high capacity fork lift.
There is a need for a relatively small, lightweight, modular tank, either above ground or buried, which can be transported to the installation site over narrow, unimproved roads using light to medium duty equipment. At the site, it should be possible to easily unload and quickly connect several tanks together to provide a larger storage system. Ideally, the system should provide various configurations which are adaptable to the terrain or needs of the user. Alignment between the various modules must not be critical. It should be possible to install and connect the modules using unskilled labor and materials which are commonly available in rural areas. Ideally, it should be possible to periodically inspect, and if necessary repair, the inter-tank connections after installation of the system without excavation. There is also a need for a similar modular system of constructing vehicle mounted tanks for transporting fluids, where the individual modules are easily installed and removed, preferably by one person with minimal equipment.
The present invention is directed to an apparatus for the storage of liquids, primarily water, in an underground tank. The tank is adapted for use as a module in a larger storage system comprised of multiple tanks.
According to the invention there is provided a tank with a ribbed main body and curved end caps and at least one fitting which extends inward into the cavity of the tank. The ribs alternate between outward circular ribs, and inward octagonal ribs. Where the tanks will be positioned immediately adjacent to each other, the tank can be manufacture with a flat panel on each end and the fitting positioned within the panel. The fitting is initially formed as a fluid tight, integrally molded cup and the end of the cup is cut out when needed to connect to a pipe.
According to an aspect of the invention, the shape of the tank is that of a modified sphere, with the diameter and length of the tank being substantially the same.
According to another aspect of the invention, a lengthwise rib may be added to further stiffen the structure or the rib may encircle the tank along a lengthwise circumference.
Further in accordance with the invention, one or more alignment cups may be formed in the ends which accept short pipe segments for aligning adjacent tanks.
Still further in accordance with the invention, one or more spreader tubes may be positioned within the tank, pushing outward on the ends for increased strength.
Yet further in accordance with the invention, the tanks may be configured into a system of tanks, interconnected by pipes, the pipes coupling to the tanks by means of a flexible sleeve and band clamps. The sleeve and clamps are positioned within cavity of the tank. The sleeve may be of various forms or materials as necessary to provide the required amount of flexibility in the joint. If desired, the pipe may have a smaller diameter than the fitting, increasing the tolerance for angular deflection.
The advantages of such an apparatus are a tank which is relatively small and lightweight, and easily transported to remote sites. There, the tanks can be combined into a storage system of almost unlimited capacity. The tanks may be laid out in any configuration or orientation required to adapt to the local terrain. Interconnection of the tanks is achieved with simple readily available materials and can be performed by relatively unskilled labor. The joints are within the tanks, protected from the environment and readily accessible after installation for inspection or repair without excavation.
The above and other features and advantages of the present invention will become more clear from the detailed description of a specific illustrative embodiment thereof, presented below in conjunction with the accompanying drawings.